Run library checks in parallel
If you run steam-runtime-system-info with "expectations", it checks that various libraries are compatible with the Steam Runtime versions.
Ideally we'd do these in parallel: fork 1-2 subprocesses per CPU core, and whenever one of them finishes, fork another. We don't need to use threads for this, because we can manage any number of subprocesses from one thread.
The design in #88 (closed) would be very suitable for this: we'd just use one GMainContext, and attach all of the sources to it.